1. Field of the invention
The present invention generally relates to a method of processing a substrate with plasma by, e.g., plasma-enhanced atomic layer deposition (PEALD) or other plasma-enhanced deposition, particularly to a method for stabilizing plasma ignition in such a process.
2. Description of the Related Art
In PEALD processes, for example, RF power is an important parameter for controlling film quality and characteristics. For example, high RF power can improve quality of film; however, in some applications, extremely low RF power is required. The following three applications are typical examples.
FIG. 1A is a schematic view illustrating a borosilicate glass (BSG) process for solid state diffusion (SSD). The reason that low RF power is required in this process is that if high RF power is applied, dopant implantation into a Si substrate occurs at a film-forming stage by ion bombardment. As illustrated in FIG. 1A, when a BSG film 1 is formed on a substrate 3, boron B penetrates into the substrate 3, forming a boron-diffused layer 2 in the substrate 3. It is possible to confine diffusion of such foreign matter to certain desired areas on the substrate surface by removing a BSG film while or after forming the BSG film, followed by heat treatment, so as to avoid unwanted diffusion of the foreign matter. However, if diffusion occurs during the film-forming process, foreign matter will inevitably diffuse to areas where diffusion of the foreign matter is not desired. A phosphosilicate glass (PSG) process has problems similar to the above when high RF power is applied.
FIG. 1B is a schematic view illustrating a 510 process in which oxidization of an underlying layer is not desired. The reason that low RF power is required in this present process is that if high RF power is applied, the material of the underlying layer will be oxidized at a film-forming stage by oxygen plasma, ion bombardment and/or spattering. As illustrated in FIG. 1B, when a SiO film 4 is formed on a tungsten (W) layer 6, the W layer 6 is oxidized, forming an oxidized layer 5 in the W layer 6. When an underlying layer contains materials such as W and TiN or a film such as a SiN film, and a PEALD-SiO film is deposited thereon, the material of the underlying layer is oxidized by a plasma containing an oxidizing gas for forming the PEALD-SiO film. Devices with the oxidized underlying layer do not possess desired characteristics.
FIG. 1C is a schematic view illustrating a film-forming process on a photoresist. The reason that low RF power is required in this present process is that if high RF power is applied, the photoresist will be etched or corroded at a film-forming process stage by oxygen plasma, ion bombardment and/or spattering. As illustrated in FIG. 1C, when a SiO film 7 is formed on a photoresist 8 formed on an under layer 9, the photoresist 8 is etched or corroded, diminishing the size of the photoresist 8. In a double patterning process scheme, when depositing a PEALD-oxide film on a photoresist, a plasma which includes oxidizing gas for forming the PEALD-oxide film oxidizes the underlying photoresist, and also sputtering shrinks the photoresist. As a result, a photoresist with a desirable size cannot be obtained.
On the other hand, application of low RF power causes problems. If RF power is low, the effects by power loss in RF transmitter circuits, sensitivity to matching box positioning, and/or sensitivity to changes in the atmosphere in a reaction chamber (e.g., due to changes in thickness of depositing films) become more serious. Accordingly, sometimes voltage does not reach the firing voltage, causing plasma ignition failure or shortening plasma treatment. As a result, the expected processing results (e.g., film thickness or uniformity) cannot be obtained.
Any discussion of problems and solutions involved in the related art has been included in this disclosure solely for the purposes of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion were known at the time the invention was made.